1. Field of the Invention
The present invention relates to a vehicle lighting apparatus, and more particularly to a vehicle lighting apparatus that controls the lighting of a semiconductor light source composed of semiconductor light emitting elements.
2. Related Art
Conventionally, vehicle lighting fixtures using a semiconductor light emitting element, such as an LED (Light Emitting Diode), as a light source is well known. Vehicle lighting fixtures of this type have a lighting control circuit for controlling the lighting of the LED.
A lighting control circuit using a switching regulator capable of controlling the output voltage to the LED based on the current of the LED is well known. The switching regulator can control the output voltage to flow a specified current through each LED, even if a plurality of LEDs are connected in series or parallel as the load.
However, if the output of the switching regulator is short circuited, or has a ground fault, the switching regulator has a heavier load, and may fail with the excessive power load. Also, if the output of the switching regulator is opened due to a disconnection in the LED, the output voltage may be increased excessively, for example, in a switching regulator of a flyback type.
Thus, a switching regulator has been proposed in which, when a failure of the switching regulator on the output side is detected, the output voltage of the switching regulator is lowered (refer to patent document 1).    [Patent document 1] JP-A-2004-134147 (pages 3 to 8, Figs. 1, 6 and 7).
In the above prior art, a plurality of LED blocks are connected in parallel on the output side of the switching regulator, a series regulator comprising a switch element, a shunt resistor and a comparator being connected in series to each LED block, and the output voltage of the switching regulator is controlled so that the current of each LED block may become a specified value, in which the output voltage of the switching regulator is lowered when the failure occurs, whereby the operating LEDs can be protected even if some LEDs fail. However, it is not fully considered to precisely detect the failure in the individual LEDs making up the LED block.
For example, when the forward voltage Vf of each LED is detected to make the failure detection of each LED in three LED blocks connected in series due to a drop in the forward voltage Vf of each LED, it is necessary to consider the variations in the forward voltage Vf of the LED, because the forward voltage Vf of the LED has variations due to the “individual difference of LED”, the “temperature characteristic of Vf” and the “V-I characteristic”. Further, in detecting the forward voltage of each LED, when the voltage across each LED is divided and each partial voltage obtained by dividing the voltage is amplified by a differential amplification circuit, for example, assuming that the forward voltage Vf of three LEDs is about 10V, a voltage of 30V is applied across the three LEDs on the basis of the reference potential (ground=0V), in which the voltages of 30V and 20V are applied across the LED on the high potential side, the voltages of 20V and 10V are applied across the LED in the middle, and the voltages of 10V and 0V are applied across the LED on the reference potential side.
That is, 10V in 30V is detected and amplified by the differential amplification circuit to detect the forward voltage Vf of the LED on the high potential side, whereas 10V in 10V is detected and amplified by the differential amplification circuit to detect the forward voltage Vf of the LED on the reference potential side.